U.S. patent number 10,894,157 [Application Number 15/933,158] was granted by the patent office on 2021-01-19 for laser marking of raw anode foil to induce uniform pattering and etching with oxide passivation mask.
This patent grant is currently assigned to Pacesetter, Inc.. The grantee listed for this patent is Pacesetter, Inc.. Invention is credited to David R. Bowen, Kurt J. Erickson, Ralph Jason Hemphill.
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United States Patent |
10,894,157 |
Bowen , et al. |
January 19, 2021 |
Laser marking of raw anode foil to induce uniform pattering and
etching with oxide passivation mask
Abstract
A method of producing an electrode for use in the manufacture of
electrolytic capacitors for implantable cardioverter defibrillators
comprises first growing a hydrate layer and/or formed oxide layers
of the foil, applying a laser beam to portions of the foil to
ablate the aluminum oxide foil surface before etching in order to
induce etching in the specific areas, and then, etching the foil.
The laser marks the oxide layer in a pulsed spot pattern through
the hydrate layer and/or formed oxide layer leaving a dimpled
nascent aluminum surface. The oxide layer left behind is a mask and
the fresh aluminum areas are the high etching activation sites.
After marking the aluminum oxide foil surface, the foil may be
electrochemically etched in an electrolyte containing chloride
and/or various oxidative species.
Inventors: |
Bowen; David R. (Taylors,
SC), Hemphill; Ralph Jason (Sunset, SC), Erickson; Kurt
J. (Anderson, SC) |
Applicant: |
Name |
City |
State |
Country |
Type |
Pacesetter, Inc. |
Santa Clara |
CA |
US |
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Assignee: |
Pacesetter, Inc. (Santa Clara,
CA)
|
Appl.
No.: |
15/933,158 |
Filed: |
March 22, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180280689 A1 |
Oct 4, 2018 |
|
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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62481433 |
Apr 4, 2017 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23K
26/082 (20151001); B23K 26/359 (20151001); H01G
9/0029 (20130101); B23K 26/361 (20151001); B23K
26/073 (20130101); H01G 9/055 (20130101); H01G
9/0032 (20130101); H01G 9/045 (20130101); B23K
26/362 (20130101); B23K 26/0624 (20151001); A61N
1/0563 (20130101); A61N 1/3975 (20130101); A61N
1/3956 (20130101) |
Current International
Class: |
B23K
26/00 (20140101); B23K 26/073 (20060101); H01G
9/045 (20060101); B23K 26/0622 (20140101); B23K
26/082 (20140101); A61N 1/05 (20060101); H01G
9/00 (20060101); B23K 26/361 (20140101); B23K
26/359 (20140101); H01G 9/055 (20060101); B23K
26/362 (20140101); A61N 1/39 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ahmed; Shamim
Parent Case Text
PRIORITY
The present application relates to and claims priority from U.S.
provisional patent application Ser. No. 62/481,433, filed Apr. 4,
2017, entitled "LASER MARKING OF RAW ANODE FOIL TO INDUCE UNIFORM
PATTERING AND ETCHING WITH OXIDE PASSIVATION MASK," which is hereby
expressly incorporated by reference in their entirety to provide
continuity of disclosure.
Claims
What is claimed is:
1. A method of producing an electrode for a capacitor from a foil
comprising: growing a hydrate layer on a surface of a foil;
applying a laser beam to the hydrate layer so as to remove the
hydrate layer from the foil, the laser beam being applied after the
hydrate layer is grown on the foil; and etching the foil after the
laser beam is applied to the portion of the foil, a second portion
of the hydrate layer being present on the foil while etching the
foil.
2. The method of claim 1, wherein the laser beam is applied so as
to ablate the portion of the hydrate layer.
3. The method of claim 1, wherein the laser beam passes through the
hydrate layer to the underlying foil so as to create a pattern on
the surface of the foil.
4. The method of claim 3, wherein the laser beam is applied such
that the laser beam forms dimples in the foil that penetrate the
surface of the foil to a depth of approximately 0.1 micron to
approximately 20 micron.
5. The method of claim 1, wherein etching the foil includes placing
the foil in an electrochemical bath comprising a chloride
electrolyte and heating said electrolyte to about 85.degree. C.
6. The method of claim 5, wherein the foil is etched in the bath at
a current density of about 0.10-0.25 Amp/cm.sup.2.
7. The method of claim 1, wherein the laser beam is a femtosecond
laser.
8. The method of claim 3, wherein the pattern is a hexagonal
pattern.
9. The method of claim 1, wherein the laser beam is applied at a
scan rate in the range of about 50 m/sec.
10. The method of claim 1, wherein the laser beam is applied with a
spot size in a range of about 1 to about 10 microns.
11. The method of claim 1, further comprising: growing an oxide
layer on a surface of the foil, and wherein the laser beam is
applied so as to remove select portions of the hydrate layer and
the oxide layer from the surface of the foil.
12. The method of claim 1, wherein the laser beam is applied while
the hydrate layer is present on the surface of the foil.
13. The method of claim 1, wherein the hydrate layer is formed
below the surface of the foil and the laser beam is applied so as
to expose a portion of the foil located below the hydrate
layer.
14. The method of claim 1, wherein a portion of the hydrate layer
is present on the foil after the laser beam is applied.
15. The method of claim 1, wherein etching the foil is performed
such that the hydrate layer acts as a mask during the etch.
16. A method of producing an electrode for a capacitor from a foil
comprising: growing a hydrate layer on a surface of the foil;
applying a laser beam to the hydrate layer so as to remove portions
of the hydrate layer from the foil; and etching the foil after the
laser beam is applied.
17. The method of claim 16, wherein the removed portions of the
hydrate layer are arranged in a spot pattern with the spots each
penetrating completely through the hydrate layer.
18. The method of claim 17, wherein the laser beam is applied such
that the laser beam only partially penetrates through the foil to
from a plurality of dimples on the foil surface.
19. The method of claim 18, wherein the laser beam is applied using
a laser head having a scan rate of 50 m/sec and the dimples have a
linear pitch of 1 micron per line.
20. The method of claim 16, wherein the laser beam is applied so as
to mark the hydrate layer and/or oxide layer with spots arranged in
a hexagonal array pattern.
21. The method of claim 16, wherein the laser beam is applied in a
femtosecond time frame using a pulse repetition rate of
approximately 50 MHz.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention is directed to a method of producing an
electrode for use in the manufacture of electrolytic capacitors and
more particularly to a method of creating porous electrode foil for
use in multiple electrode stack configuration electrolytic
capacitors of the type used in implantable cardioverter
defibrillators (ICDs).
Related Art
Compact, high voltage capacitors are utilized as energy storage
reservoirs in many applications, including implantable medical
devices. These capacitors are required to have a high energy
density since it is desirable to minimize the overall size of the
implanted device. This is particularly true of an implantable
cardioverter defibrillator (ICD), also referred to as an
implantable defibrillator, since the high voltage capacitors used
to deliver the defibrillation pulse can occupy as much as one third
of the ICD volume.
Electrolytic capacitors are used in ICDs because they have the most
near ideal properties in terms of size and ability to withstand
relatively high voltage. Conventionally, an electrolytic capacitor
includes an etched aluminum foil anode, an aluminum foil or film
cathode, and an interposed kraft paper or fabric gauze separator
impregnated with a solvent-based liquid electrolyte. The
electrolyte impregnated in the separator functions as the cathode
in continuity with the cathode foil, while an oxide layer on the
anode foil functions as the dielectric. The entire laminate is
rolled up into the form of a substantially cylindrical body, or
wound roll, that is held together with adhesive tape and is
encased, with the aid of suitable insulation, in an aluminum tube
or canister. Connections to the anode and the cathode are made via
tabs. Alternative flat constructions for aluminum electrolytic
capacitors are also known, composing a planar, layered, stack
structure of electrode materials with separators interposed
therebetween.
Since these capacitors must typically store approximately 30-40
joules, their size can be relatively large, and it is difficult to
package them in a small implantable device. A patient who has a
device implanted may often be bothered by the presence of the large
object in his or her pectoral region. Furthermore, the generally
rectangular shape can in some instances lead to pocket erosion at
the somewhat curved corners of the device. For the comfort of the
patient, it is desirable to make smaller and more rounded ICDs. The
size and configuration of the capacitors has been a major stumbling
block in achieving this goal.
In ICDs, as in other applications where space is a critical design
element, it is desirable to use capacitors with the greatest
possible capacitance per unit volume. Since the capacitance of an
electrolytic capacitor increases with the surface area of its
electrodes, increasing the surface area of the aluminum anode foil
results in increased capacitance per unit volume of the
electrolytic capacitor. By electrolytically etching aluminum foils,
an enlargement of a surface area of the foil will occur. As a
result of this enlargement of the surface area, electrolytic
capacitors, which are manufactured with the etched foils, can
obtain a given capacity with a smaller volume than an electrolytic
capacitor.
In a conventional electrolytic etching process, surface area of the
foil is increased by electrochemically removing portions of the
foil to create etch dimples. For example, U.S. Pat. Nos. 4,474,657,
4,518,471 and 4,525,249 to Arora, each of which is incorporated
herein by reference, disclose the etching of aluminum electrolytic
capacitor foil by passing the foil through an electrolyte bath. The
preferred bath contains 3% hydrochloric acid and 1% aluminum as
aluminum chloride. The etching is carried out under a direct
current (DC) and at a temperature of 75.degree. C. U.S. Pat. No.
4,474,657 is limited to the above single step. U.S. Pat. No.
4,518,471 adds a second step where the etched foil is treated in a
similar bath with a lower current density and at a temperature of
80-82.5.degree. C. U.S. Pat. No. 4,525,249 adds a different second
step, where the etched foil is treated in a bath of 8% nitric acid
and 2.6% aluminum as a nitrate, at a temperature of 85.degree.
C.
U.S. Pat. No. 7,452,473, incorporated herein by reference,
discloses a method of producing an electrode for a capacitor from a
foil. The method comprises, first, applying a laser beam to
portions of the foil to create a pattern on the foil and, second,
etching the foil. Patterns formed include patterns such as
horizontal or vertical lines, a grid pattern, a crossed-grid
pattern, a parallel wave pattern, an intersecting wave pattern, a
labyrinth pattern, a staggered hole pattern, or alternatively an
irregular pattern.
U.S. Pat. No. 8,734,964 to Kurihara, incorporated herein by
reference, discloses laminating an etching resist over a molded
material or sequentially laminating a first heat-generating layer,
a second heat-generating layer, and a metal compound layer
including a metallic oxynitride layer containing a metallic
oxynitride over a molded material. The first heat-generating layer,
the metallic oxynitride layer, and the second heat-generating layer
are directly or indirectly laminated such that the metallic
oxynitride layer is positioned between the first heat-generating
layer and the second heat-generating layer. A laser beam is
irradiated over the laminated etching resist to form penetrating
holes in the etching resist.
There remains a need for an improved method for etching anode
foil.
SUMMARY OF THE INVENTION
The present invention is directed to a method of producing an
electrode for a capacitor from a foil. The method comprises, first,
growing a hydrate layer and/or formed oxide layers of the foil,
applying a laser beam to portions of the foil to ablate the
aluminum oxide foil surface before etching in order to induce
etching in the specific areas, and then, etching the foil. In
certain embodiments, the laser marks the oxide layer in a pulsed
spot pattern through the hydrate layer and/or formed oxide layer
leaving a dimpled nascent aluminum surface. The oxide layer left
behind is a mask and the fresh aluminum areas are the high etching
activation sites. After marking the aluminum oxide foil surface,
the foil may be electrochemically etched in an electrolyte
containing chloride and/or various oxidative species. By
appropriately spacing the laser spots in a close packed, e.g.,
hexagonal array, the foil capacitance can be maximized by causing
pattern etching of the foil surface. In certain embodiments, the
laser pulse is delivered in the femtosecond time frame in order to
increase the speed of marking and to provide an advantageously high
ratio of peak to average power which maximizes ablation efficiency
of the protective oxide layer while essentially eliminating
thermally induced damage of the desirably engineered cubic
structure of the underlying aluminum substrate. In certain
embodiments, a `femtosecond class` laser is used.
It is to be understood that both the foregoing general description
and the following detailed description are exemplary and
explanatory and are intended to provide further explanation of the
invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
The accompanying drawings, which are included to illustrate
exemplary embodiments of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and together with the description serve to explain
the principles of the invention. In the drawings:
FIG. 1A is a Scanning Electron Microscope (SEM) photograph of an
aluminum foil electrochemically etched and widened using
conventional methods.
FIG. 1B is shows a graph highlighting the effects of conventional
etching processes on foil properties.
FIG. 1C shows results measuring capacitance verses dimple spacing
in a close-packed hexagonal array for different etching radii for a
450 Volt EFV (effective formation voltage) foils.
FIGS. 2A-2C are flow diagrams showing methods of producing an
electrode with, first, growing a hydrate layer and/or formed oxide
layers of the foil, and applying a laser beam to portions of the
foil to ablate the aluminum oxide foil surface before etching in
order to induce etching in specific areas.
FIG. 3A illustrates an optical arrangement for generating a laser
pulse, according to an embodiment.
FIG. 3B illustrates a comparison between a Gaussian and a top-hat
spatial profile.
FIG. 3C illustrates a portion of the optical arrangement for
generating multiple beam spots, according to an embodiment.
FIG. 4A illustrates a configuration for controlling laser ablation
on a metal foil, according to an embodiment.
FIG. 4B illustrates an arrangement of dimples on a metal foil
formed via the configuration of FIG. 3A, according to an
embodiment.
FIG. 4C illustrates a close packed hexagonal array arrangement of
dimples, according to an embodiment.
FIG. 5 illustrates a flow chart of a method of forming dimples on a
metal foil, according to an embodiment.
DETAILED DESCRIPTION OF THE INVENTION
In the detailed description that follows, references to "one
embodiment," "an embodiment," "an illustrative embodiment," etc.,
indicate that the embodiment described may include a particular
feature, structure, or characteristic, but every embodiment may not
necessarily include the particular feature, structure, or
characteristic. Moreover, such phrases are not necessarily
referring to the same embodiment. Further, when a particular
feature, structure, or characteristic is described in connection
with an embodiment, it is submitted that it is within the knowledge
of one skilled in the art to affect such feature, structure, or
characteristic in connection with other embodiments whether or not
explicitly described.
It should be understood that not all embodiments of the invention
include the discussed features, advantages or modes of operation.
Alternate embodiments may be devised without departing from the
scope of the invention, and well-known elements of the invention
may not be described in detail or may be omitted so as not to
obscure the relevant details of the invention. In addition, the
terminology used herein is for the purpose of describing particular
embodiments only and is not intended to be limiting. For example,
as used herein, the singular forms "a", "an" and "the" are intended
to include the plural forms as well, unless the context clearly
indicates otherwise. It will be further understood that the terms
"comprises," "comprising," "includes" "including" and/or "having,"
when used herein, specify the presence of stated features, steps,
operations, elements, and/or components, but do not preclude the
presence or addition of one or more other features, steps,
operations, elements, components, and/or groups thereof.
In accordance with the present disclosure, improved anode foils and
methods of fabricating these improved structures are provided.
Various advantages and benefits accrue for the embodiments
disclosed herein. An elaboration of some of these advantages and
benefits is provided below. It is noted that the following list of
advantages and benefits is for purposes of illustration and not
limitation.
The techniques, materials and methods disclosed herein may be used,
for example, to manufacture anodes, anode stacks and capacitors
containing such anodes and anode stacks, such as those disclosed in
U.S. Pat. No. 7,130,183 to Fayram and/or U.S. Pat. No. 7,196,899 to
Feger et al., both of which are incorporated herein by reference as
if reproduced in full below.
The method comprises growing a hydrate layer and/or formed oxide
layers on a metal foil surface and using a laser to ablate the
metal foil surface before etching in order to induce etching in
specific areas. By appropriately spacing the laser spots in a close
packed array, the foil capacitance can be maximized by causing
pattern etching of the foil surface that increase the surface area
of the metal foil. The foil can be used as anodes, which in turn
are used in assembling a capacitor. The increased surface area for
the anodes may equate to a reduction in overall capacitor volume of
about 50-66%, which in terms of current ICD volume is a 9 to 12 cc
reduction. Or alternatively, fewer anodes may be needed to deliver
the same energy output, thereby reducing production costs of the
capacitor.
Growing Hydrate Layer
FIG. 2A is a high level flowchart of an exemplary method 200a of
imparting a pattern on a metal foil surface. The method 200a can
include step 202 of forming a hydrate layer on a raw metal foil. In
certain embodiments, the metal foil is aluminum foil. In certain
embodiments, the metal foil is tantalum foil. In some embodiments,
a hydrate layer can be created by dipping the surface of metal
substrate in a bath of water. Water reacts with the exposed areas
of metal substrate to form a hydrate layer above and below the
surface of metal substrate. The hydrate layer thus forms a mask
above and below the surface of metal substrate thereby forming a
layer of protection that extends below the surface of a metal
substrate.
The following is an exemplary process for creating a hydrate layer
when an aluminum foil is used as the metal substrate. The aluminum
foil can be dipped in 95.degree. C. deionized (DI) water for 4 to
12 minutes, preferably 8 minutes, to form aluminum hydroxide above
and below the surface of the aluminum foil. In some embodiments,
the temperature of the water is in a range from 70 to 100 degrees
Celsius. The length of time to form the hydrate layer of a desired
thickness is determined in part by the temperature as the higher
the temperature, the shorter the length of time to form a hydrate
layer, as would be appreciated by one of ordinary skill in the art.
The hydrate layer may have a thickness of preferably from 7 to 200
nm, more preferably from 10 to 100 nm, and even more preferably
from 15 to 70 nm.
Oxide Layer Formation
FIG. 2B is a high level flowchart of an alternative exemplary
method 200b of imparting a pattern on a metal foil surface. The
method 200b can include step 204a of forming an oxide layer on a
raw metal foil. A barrier oxide layer may be electrochemically
formed onto the surface of the metal foil by placing the foil into
a forming solution, including but not restricted to a solution
based on azelaic acid, sebacic acid, suberic acid, adipic acid,
dodecanedioic acid, citric acid or other related organic acids and
salts, preferably a citric acid solution at a temperature of
80.degree. C. to 100.degree. C., preferably 85.degree. C., at a
current density of 1 mA/cm.sup.2 to 40 mA/cm.sup.2, preferably 16
mA/cm.sup.2. The oxide layer may have a thickness of preferably
from 7 to 200 nm, more preferably from 10 to 100 nm, and even more
preferably from 15 to 70 nm.
Forming Hydrate and Oxide Layers
FIG. 2C is a high level flowchart of an alternative exemplary
method 200c of imparting a pattern on a metal foil surface. The
method 200c can include both step 202 of forming a hydrate layer on
a raw metal foil and step 204b of forming an oxide layer onto
surfaces of the metal foil.
Applying Laser Beam
At step 206 of method 200a (FIG. 2A), method 200b (FIG. 2B), and
method 200c (FIG. 2C), a laser beam is applied to portions of the
foil to ablate the metal foil, e.g., aluminum oxide foil, surface
before etching in order to induce etching in specific areas. In
certain embodiments, the laser marks the oxide layer in a pulsed
spot pattern through the hydrate layer, leaving a dimpled nascent
aluminum surface. The hydrate layer left behind is a mask and the
fresh aluminum areas are the high etching activation sites.
Advantageously, unlike a laminate layer disclosed in U.S. Pat. No.
8,734,964, which simply sits on the surface of the molded material,
the hydrate layer of the current disclosure incorporates into the
metal foil, e.g. the aluminum foil. Because the hydrate of the
current disclosure incorporates into the metal foil, it does not
peel off during etching, enhancing its effectiveness as a mask
during the etching process.
The desired pulsed laser ablated spots are a diameter of about 1 to
10 microns with about 2 to 20 micron center to center spacing in a
hexagonal close packed array. The depth of the laser mark should be
through the hydrate layer to expose the fresh surface, e.g.,
aluminum foil, then 0.5 to 20 microns deep into the metal foil,
without heating the substrate such that the cubic structure is
damaged, or a protective thermal oxide is formed in the mark zone.
In embodiments of the current disclosure, the laser does not
penetrate through the substrate but instead merely provides dimples
in the metal foil. Use of a laser pulse delivered in the
femtosecond time frame is greatly preferred due to the speed of
marking, and the phenomenally high ratio of peak to average power
which maximizes ablation efficiency of the protective oxide layer
while essentially eliminating thermally induced damage of the
desirably engineered cubic structure of the underlying substrate,
e.g., aluminum or tantalum substrate.
Example Laser Source
FIG. 3A illustrates an optical arrangement 300 that includes a
laser source 302, a beam shaping element 306, and a focusing
element 308, according to an embodiment. Laser source 302 may
include a pulsed laser having a wavelength between about 200 nm and
1070 nm. Laser source 302 may be designed as an ultrashort pulse
source (i.e., having a picosecond or femtosecond pulse width). Use
of a laser pulse delivered in the femtosecond time frame is
preferred due to the speed of marking, and the phenomenally high
ratio of peak to average power which maximizes ablation efficiency
of the protective oxide layer while essentially eliminating
thermally induced damage of the desirably engineered cubic
structure of the underlying aluminum substrate.
In an embodiment, optical arrangement 300 is provided within a
laser head that may be movable to ablate various portions of a
sample. In another embodiment, only some components of optical
arrangement 300 are provided within the laser head. For example,
the laser head may only include focusing element 308.
Laser source 302 outputs a beam of light having a substantially
Gaussian spatial profile 304. Although this beam of light may be
used for ablation, it may be more difficult to control the ablation
depth and/or volume of the beam spot given the energy distribution
of Gaussian profile 304. This is discussed in more detail later
with reference to FIG. 3B.
According to an embodiment, the beam of light with Gaussian profile
304 is received by a beam shaping element 306 and converted to a
beam of light having a top hat spatial profile 310. For example,
the beam of light with Gaussian profile 304 may have an M.sup.2
value less than 1.2 while the beam of light having top hat profile
310 may have an M.sup.2 value greater than 20. The M.sup.2 value is
a parameter that quantifies the beam quality of a laser beam. An
ideal Gaussian beam has an M.sup.2 value of 1, while higher M.sup.2
values have a more uniform energy profile. Beam shaping element 306
may be a fused silica diffractive optic designed to modulate the
energy profile of an input Gaussian beam. One example of beam
shaping element 306 is the FBSR Beam Shaper from Topag
Lasertechnik.
It should be understood that Gaussian profile 304 and top hat
profile present a cross-sectional energy distribution of the laser
beam at their respective locations along the optical path. The beam
of light produced by laser source 302 may be characterized as
having Gaussian profile 304 up until it is received by beam shaping
element 306, after which the beam of light may be characterized as
having top hat profile 310. In other embodiments, beam shaping
element 306 is not included and the laser light characterized as
having Gaussian profile 304 is focused by focusing element 308 onto
a beam spot for ablation of a dimple through a metal foil.
The beam of light having the top hot profile 310 may then be
focused onto a beam spot using focusing element 308. FIG. 3A
illustrates how the focused beam of light has the top hat profile
310 due to beam shaping element 306. Focusing element 308 may
represent one or more lenses arranged to focus the collimated light
beam onto a beam spot for ablation of a dimple onto a metal foil,
according to an embodiment.
FIG. 3B illustrates how the size of the resulting beam spot changes
based on applied power density and energy profile of the beam. An
ablation threshold 312 is indicated, which represents the power
needed to start ablating through the hydrate/oxide layer and
forming a dimple on the surface of the metal, without penetrating
through the metal. As the beam spot is focused to include a
narrower region of Gaussian profile 314, there is a high power
output at the center of the beam spot and lower power output at the
edges of the beam spot. Although ablation can occur using this
energy profile, the resulting ablation morphology, e.g., depth
and/or volume is more difficult to control, and the transfer of
heat to the material is more significant, due to the substantial
change in delivered power from the center of the beam spot to the
edge of the beam spot.
At ablation threshold 312, top hot profile 316 provides a more
uniform energy distribution across the beam spot, even as the beam
spot is focused to include a narrower region of top hat profile
316. Thus, the ablation depth and/or volume is easier to control,
has higher repeatability, and a better overall profile than with
Gaussian profile 314.
Only one output beam of light is illustrated in optical arrangement
300. However, in another embodiment, one or more optical elements
may be included to split the beam of light having the top hat
profile 310 into multiple beams of light that are each focused onto
a metal foil at different beam spots. Beam splitting in this way
may be achieved with diffractive optics to produce a linear or 2D
array of beam spots having equal or nearly equal power densities.
FIG. 3C illustrates an example of a diffractive splitting element
318, that may be used to split an incoming beam of light 320 into a
plurality of light beams 322a-322c. Although only three light beams
are illustrated here, it should be understood that any number of
light beams may be generated. Diffractive splitting element 318 may
be located optically downstream from shaping element 306 (if one is
used), but optically upstream from focusing element 308.
Roller-Based Laser Drilling Technique
FIG. 4A illustrates a sheet roller arrangement 400 for ablating
dimples on the surface of a metal foil 402, according to an
embodiment. Metal foil 402 may be used as an anode in an
electrolytic capacitor, and may be an aluminum or tantalum foil. A
laser head 404 outputs a beam of light 406 having a top hat profile
at a beam spot towards a surface of metal foil 402. The term "beam
spot" as used herein refers to a location of focused light from the
perspective of the laser head. Laser head 404 may have an internal
arrangement similar to optical arrangement 300. In one embodiment,
more than one beam of light having a top hat profile is outputted
from laser head 404.
Metal foil 408 is stretched over a roller 408 such that a foil
portion 403 faces laser head 404, according to an embodiment. Metal
foil 402 may be stretched over roller 408 at any angle.
Furthermore, metal foil 402 may be placed around other rollers or
spooled around other rollers that are not shown for clarity. The
arrows provide an example movement of metal foil 402 around roller
408. With respect to laser head 404, metal foil would appear to
move upward. This direction is arbitrary, and it should be
understood that other examples may have metal foil 402 moving
downward with respect to laser head 404 and around roller 408 in a
counter-clockwise direction. Laser head 404 may be designed to scan
in a direction that is perpendicular to the direction that metal
foil 402 moves around roller 408 (as viewed from laser head 404),
such that beam of light 406 ablates dimples across a length of foil
portion 403 (normal to the page in this illustration). This
ablation method is illustrated in more detail in FIG. 4B.
FIG. 4B illustrates a view of metal foil 402 as viewed from laser
head 404, according to an embodiment. Metal foil 402 includes a
plurality of etched dimples 410 that are arranged in rows. Three
example rows 412a-412c are illustrated in FIG. 4B, however, it
should be understood that any number of rows of dimples may be
ablated in metal foil 402. In the illustrated example, laser head
404 is scanned in a first direction 405 across a length of metal
foil 402 and pulses energy towards metal foil 402 at a given beam
spot to form a dimple 410. The laser power density being applied is
above an ablation threshold of the metal foil. For example, the
laser power density being applied may be between 7.8.times.10.sup.8
W/cm.sup.2 and 4.times.10.sup.10 W/cm.sup.2. In another example,
the laser power density being applied is greater than
7.8.times.10.sup.8 W/cm.sup.2. In embodiments, the pulse repetition
rate of the laser is approximately 50 MHz, and the scan head runs
at the rate of 50 m/sec, such that the pits or dimples have a
linear pitch of 1 micron per line. If the length of the scan line
is 200 mm, the vertical scan rate is 500 lines/second. If the
vertical pitch is 1 micron, then the web speed would be 500
microns/sec, or 30 mm/Min. In an embodiment, a pattern may be
marked in 5.33 minutes per aluminum metal sheet of 320 cm.sup.2
area.
Once a dimple has been formed, laser head 404 is laterally scanned
in the first direction 405 and pulses energy towards metal foil 402
at a new beam spot to form the next dimple.
In the illustrated example, the entire first row 412a of dimples
410 have been ablated by beam of light 406. At this point, metal
foil 402 may be moved in a second direction 407, such that the next
row 412b is present as foil portion 403. The laser then proceeds to
form dimples in row 412b.
The example illustrated in FIG. 4B shows that the laser has formed
dimples partially along the length of row 412b, but has not yet
formed dimples at locations 414 marked with broken circles.
Additionally, the entire next row 412c includes all locations 414
where dimples have not yet been formed. Once each location 414
across row 412b has been ablated to form a dimple, metal foil 402
may again be moved in the second direction 407 to present row 412c
as foil portion 403 to have its locations 414 ablated to form more
dimples.
It should be understood that many alterations may be made to how
various elements move and when ablation takes place without
deviating from the scope or sprit of the embodiments described
herein. For example, laser head 404 may scan along the length of
metal foil 402 in the first direction 405, forming dimples at each
beam spot as it does, and then scan back across the length of metal
foil 402 in the opposite direction to form dimples in the next row.
In another example, roller 408 is laterally shifted in the first
direction 405 while laser head 404 remains stationary. Laser head
404 may be controlled to form dimples 410 having any desired
arrangement. For example, the packing density of dimples 410 may be
a close packed hexagonal array arrangement, as illustrated in FIG.
4C. The overall dimple density may be on the order of 20 to 50
million dimples per cm.sup.2. Furthermore, the diameters of the
dimples may be changed by altering the beam spot size. Since the
beam has a top-hat energy profile, changing the beam spot size
would not adversely affect the ability for the laser beam to punch
through a portion of the thickness of the foil and form the dimple.
In certain embodiments, the dimple is 0.5 to 20 microns deep. In
another example, larger diameter dimples may be formed by
clustering beam spots close enough together that they overlap.
In an embodiment, multiple beam spots may be formed on the surface
of metal foil 402 along a length of foil portion 403. In this way,
multiple dimples may be ablated at substantially the same time. In
the example illustrated, the first four dimples of row 412b may be
ablated at the same time by projecting four beams of light and
creating four beam spots linearly arranged in the first direction
405. Providing multiple beams would substantially decrease the time
it takes to ablate dimples across the surface of metal foil
402.
FIG. 5 provides an example method 500 of ablating dimples in an
anode foil. In one example, the anode foil is an aluminum foil or a
tantalum foil that is used as the anode within a capacitor or
battery. Any of the step(s) of method 500 may be performed using
sheet roller arrangement 400. Furthermore, the various steps of
method 500 may be performed in any order.
At block 502, a sheet of metal foil is stretched around a
cylindrical body. The stretching may cause a portion of the foil to
lay substantially flat across a length of the foil, where the
portion is aligned with a laser head that delivers the pulses of
light to ablate the foil.
At block 504, a first beam of light is generated. The first beam of
light may be a beam of laser light having a wavelength between 200
nm and 1070 nm. The first beam of light may have a Gaussian energy
profile and may be collimated to form a more focused beam. A
standard optical collimator may be used to collimate the laser
light generated from a source, as would be understood by those
skilled in the art.
At block 506, the Gaussian energy profile of the first beam of
light is altered to form a second beam of light having a top hat
profile. The energy profile may be altered using a beam shaping
element, such as a fused silica diffractive optic.
At block 508, the second beam of light having the top hat profile
is pulsed at a first beam spot towards the sheet of metal foil. The
second beam of light may also be focused onto the first beam spot
using one or more lenses, thus affecting the ablation depth and/or
volume. The second beam of light may have femtosecond pulses 50
MHz, running at the rate of 50 m/sec, such that the pits or dimples
have a linear pitch of 1 micron per line. If the length of the scan
line is 200 mm, the vertical scan rate is 500 lines/sec. If the
vertical pitch is 1 micron, then the web speed would be 500
microns/sec, or 30 mm/Min. 5.33 Min per sheet of 320 cm.sup.2 area
to mark the pattern.
At block 510, the second beam of light is pulsed at a second beam
spot that is displaced from the first beam spot in a first
direction. The first direction may be across a length of the metal
foil stretched around the cylindrical body. The second beam of
light is delivered from a laser head that may laterally move
between a first position, where the second beam of light is pulsed
at the first beam spot, and a second position where the second beam
of light is pulsed at the second beam spot. It should be understood
that this action may be repeated until dimples have been formed
across an entire length of the metal foil.
At block 512, the sheet of metal foil is moved in a second
direction around the cylindrical body. This second direction is
substantially perpendicular to the first direction when viewed from
the perspective of the laser head. The sheet of metal foil may move
in the second direction after dimples have been formed across a
length of the metal foil, in order to form more dimples across a
new section of the metal foil.
Etching
Referring back to FIG. 2A-C, after laser marking the foil at step
206, the foil is etched at step 208, to increase the surface area
of the foil. The foil is etched according to any method that
increases the surface area, preferably electrochemical etching.
Other methods include roughing the foil surface mechanically and
chemical etching. Electrochemical etching increases the surface
area of the foil by electrochemically removing portions of the foil
to creates etch tunnels. Electrochemical etching is done according
to any known etch process, such as the ones discussed in U.S. Pat.
Nos. 4,474,657; 4,518,471; 4,525,249 and 5,715,133, which are
incorporated herein by reference in their entirety.
The foil is etched in an electrolyte fluid that promotes
electrochemical etching. The electrolyte comprises a halide and/or
oxyhalide, preferably a chloride and/or oxychloride, and an
oxidizer such as hydrogen peroxide, sodium perchlorate, sodium
persulfate, cerium sulfate or sodium periodate. The pH of the
electrolyte is maintained in the range of about 0.1 and 9.5,
preferably between about 0.0 to about 8.0, and more preferably a pH
of about 1.0 to about 3.0. An acid is added to the electrolyte to
maintain the pH, for example hydrochloric acid. Alternative acids
for use in the present invention include but are not limited to
sulfuric, nitric, hydrobromic, and hydrofluoric acids; or organic
acids such as formic, acetic, citric and para-toluenesulfonic acid.
Other surface area enhancing etch solutions can be used with the
present invention to produce similar results. Preferably, the
electrolyte etch solution comprises about 1.3% by weight NaCl and
about 3.5% by weight NaClO.sub.4. The electrolyte is heated to a
temperature of about 80.degree. C. to about 100.degree. C., with a
preferred temperature of about 85.degree. C. The foil is placed in
the etch electrolyte and etched at a current density of about 0.1
to about 0.3 Amps/cm.sup.2, preferably about 0.15 Amps/cm.sup.2.
The current density corresponds to an etch charge of about 5 to
about 50 Coulombs/cm.sup.2 for a specific amount of time,
preferably about 36 Coulombs/cm.sup.2 for about 4 minutes. The foil
is etched to produce an enlargement of surface area of at least
about 20 times.
Upon etching, the areas where the laser was applied etch more
slowly or not at all. The hydrate layer and/or formed oxide layer
adds more etch resist masking to methods using laser marking alone,
e.g., the methods disclosed in U.S. Pat. No. 7,452,473,
incorporated herein by reference in its entirety. The enhanced etch
resist masking causes the fresh aluminum surface pits created by
the femtosecond laser to be even higher energy density areas. The
higher energy density areas promote the initiation and propagation
of the etching dimples. The hydrate/formed oxide layer inhibit
etching where present due to their electrical insulating and
relative chemical inactivity properties in the etch solution. This
passivates incidental high energy sites like rolling lines.
In certain embodiments, chemical treatment with sodium hydroxide is
used to further expose the fresh aluminum surface pits.
At step 210 (FIGS. 2A-2C), after the foil is etched, the foil is
widened in a chloride or nitrate containing electrolyte solution
known to those skilled in the art, such as that disclosed in U.S.
Pat. Nos. 3,779,877 and 4,525,249, which are incorporated herein by
reference. The foil is then dipped into a deionized water bath at a
temperature of 80.degree. C. to 100.degree. C., preferably
95.degree. C., to form a hydrate on the foil surface.
A barrier oxide layer may be electrochemically formed onto the
surface of the metal foil, sufficiently thick to support the
intended use voltage, by placing the foil into a forming solution,
including but not restricted to a solution based on azelaic acid,
sebacic acid, suberic acid, adipic acid, dodecanedioic acid, citric
acid or other related organic acids and salts. Preferably a citric
acid is used solution at a temperature of about 80.degree. C. to
100.degree. C., preferably 85.degree. C., at a current density of
about 1 mA/cm.sup.2 to 40 mA/cm.sup.2, preferably 16 mA/cm.sup.2. A
formation voltage of about 50 to 800 Volts, preferably 445 V, can
be applied to the foil to form the barrier oxide layer. The barrier
oxide layer provides a high resistance to current passing between
the electrolyte and the metal foils, also referred to as the
leakage current. A high leakage current can result in poor
performance and reliability of an electrolytic capacitor. In
particular, a high leakage current results in a greater amount of
charge leaking out of the capacitor once it has been charged.
A heat treatment of about 500.degree. C..+-.20.degree. C. may be
applied to the foil following formation for about 1 to about 10
minutes, preferably about 4 minutes. The foil is then returned to
the forming solution and allowed to soak with no applied potential
for about 1 to about 10 minutes, preferably about 2 minutes. A
second formation in the same electrolytic forming solution at high
temperature is performed at a potential of about 435 Volts.
Next, the foils may be dipped in a suitable low concentration
oxide-dissolving acid solution including but not restricted to
phosphoric acid, formic acid, acetic acid, citric acid, oxalic
acid, and acids of the halides. Preferably phosphoric acid is used
at a concentration of about 1% to 10%, preferably a concentration
of about 2%, at a temperature of about 60.degree. C. to 90.degree.
C., preferably about 70.degree. C., for a time of about one to
about ten minutes, preferably about four minutes.
Finally, the foils are reformed at a voltage of about 435 Volts in
a suitable forming solution, as discussed above, at a high
temperature, preferably about 80.degree. C. to about 100.degree.
C., more preferably about 85.degree. C.
Electrolytic capacitors manufactured with anode foils etched
according to the present invention may be utilized in ICDs, such as
those described in U.S. Pat. No. 5,522,851 to Fayram, incorporated
herein by reference. The increased capacitance per unit volume of
the electrolytic capacitor will allow for a reduction in the size
of the ICD.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the spirit and
the scope of the invention.
* * * * *